A traveling wave tube is a vacuum device which serves as an amplifier of microwave frequency energy. It relies upon the energy interaction that occurs between an electron beam and a microwave frequency signal. An electron gun at an input end of a slow wave structure (SWS) generates the electron beam. The electron beam travels along an axial path through the SWS. A microwave source inputs the microwave signal at the input end of the structure. The microwave signal then propagates along the SWS towards an output end of the SWS.
The SWS causes the microwave signal to traverse an extended distance between two axially spaced points. This reduces the effective lateral propagation velocity of the microwave signal from that of light to that of the electron beam. Interaction between the electron beam and the microwave signal causes velocity modulation and bunching of the electrons in the beam. The interaction also causes energy coupling to take place between the electron beam and the microwave signal that amplifies the signal. The amplified microwave signal is then coupled at the output end.
The amount of coupling between the electron beam and the microwave signal is approximately constant at low microwave signal input power levels. Thus, the gain between the microwave output and input signals is nearly constant. As the power of the microwave input signal increases, nonlinear effects become more significant. Eventually, the microwave output signal reaches a maximum power value and the traveling wave tube operates at saturation. Approaching saturation, the relationship between the microwave output and input signals starts to decline. If the power of the microwave input signal is increased further beyond saturation, the power of the microwave output signal and the gain decrease. A traveling wave tube operating below its saturated microwave output power is described as running backed off from saturation.
The power of the microwave output signal is also proportional to the electron beam power. Saturation of the traveling wave tube occurs, regardless of the power of the microwave input signal, when the power of the microwave output signal is roughly 5% to 50% of the electron beam power. Accordingly, for multiple signal communication applications requiring high amplitude and phase linearity, the microwave output power must be roughly 2% to 15% of the electron beam power and 5% to 50% of the saturated microwave output power.
The traveling wave tube further includes a collector for collecting the electrons in the electron beam after they have traveled through the SWS to collect the power in the beam. The power of the electron beam which has not been coupled to the microwave signal is referred to as the unused power in the spent electron beam.
Running backed off from saturation, the electrons in the electron beam have a small energy spread. Also, the velocity of the electrons remains generally axial because of minimal perturbance by the microwave signal.
Operating at saturation, the energy spread of the electron beam is large because the perturbance of the microwave signal with the beam causes some of the electrons to rapidly accelerate and decelerate. For instance, some electrons may lose as much as 50% of their initial energy while others may gain as much as 20% of their initial energy. The significant perturbance during operation at saturation, also causes the electron beam to have large radial velocity components.
Typical traveling wave tubes are built to produce the desired saturated microwave output power and then are run backed off from saturation to obtain the desired amplitude and phase linearity. This limits the performance of the traveling wave tube because the collector is designed to electrically operate at saturation. The backstreaming current, or current returned from the collector, is typically limited to less than 5% of the cathode current at saturation. In particular, the collector of a typical traveling wave tube generally has no more than four stages because the collection efficiency increases slightly with additional stages when the tube is operating at saturation. Furthermore, because the electron beam has a large energy spread and large radial velocity components at saturation, the geometry of the stages and the voltages applied to the stages are set to handle a highly divergent beam. For instance, the stages are positioned at a large angle with respect to the walls of the collector and the difference in voltages applied to the first and last stages is relatively small.
However, because the electron beam has a small energy spread and a general axial velocity during backed off operation, the efficiency of collecting the electrons may be increased with a novel collector implementation.